Integrated circuit designers have long recognized that many portable electronic systems can be improved by reducing their power dissipation or their battery requirements. Designers have also recognized that the fastest silicon transistors generally have relatively low breakdown voltages. For all of these reasons, circuit designers have sought to scale the supply voltages of digital circuits from the conventional level of about 5 volts, down to 3.3 volts, and even to 1.5 volts. However, it is difficult to preserve high-speed operation at low power-supply voltages, particularly at voltages as low as 1.5 volts.
For example, emitter-coupled logic (ECL) technology is widely used for high-speed logic gates. Depicted in FIG. 2 is a typical ECL circuit for use as a multiplexer. The output voltage, or half of the output voltage, appears at node X as a voltage drop across resistor R2.1. The current through this resistor follows alternate paths to the supply voltage V.sub.EE via transistors Q2.1 and Q2.7, or transistors Q2.3 and Q2.8. Each path includes two transistors in a stacked pair configuration. (Two transistors are said to be "stacked" if the emitter terminal of one is connected to the collector terminal of the other.) Because each base-emitter junction of these transistors generally requires a voltage drop of about 0.8 volts, the power supply voltage must be substantially greater than 1.5 volts if deep saturation of these transistors is to be avoided.
By contrast, a power supply voltage as low as 1.5 volts can be used with circuits in which no path between the power supply voltage and ground includes a stacked pair of transistors. A well-known circuit of that kind, for calculating the logical OR function, is illustrated in FIG. 1. In the circuit of FIG. 1, the flow of current through transistor Q1.3 is controlled by transistors Q1.1 and Q1.2, which are coupled to it by emitter-to-emitter connections.
Such a circuit, and circuits like it, can offer high-speed operation even at relatively low power-supply voltages, such as 1.5 volts. However, circuit designers have hitherto failed to provide more than one or a few such circuits. Until now, a general selection of circuits of this kind, able to perform a wide variety of logical operations, has been lacking.